1. Field of the Invention
The present invention relates to OLED microdisplays and more particularly to OLED microdisplays in which electrical cross-talk is greatly reduced or eliminated and to a method of making such OLED microdisplays.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
An understanding of how cross-talk affects the operation of organic light emitting diodes (OLEDs) in a microdisplay can be observed from FIGS. 1(a) and 1(b), which shows a closeup of a group of pixels in an operating display. Only three columns of pixels are actually being driven and only they should be emitting light while the pixels in between should be completely off or black. FIG. 1(b) shows a display operating correctly, with no visible cross-talk. FIG. 1(a), on the other hand, suffers from severe cross-talk since pixels adjacent to the driven column are also showing partial emission of light.
Electrical cross-talk is the occurrence of an unintentional signal generated on a pixel when an adjacent pixel is being driven, due to the parasitic capacitance that exists between the two pixels. Parasitic capacitance is always present between nearby conductors. However, it only becomes a problem when the magnitude of the parasitic capacitance results in an observable effect, as in the case of a display with visible cross-talk.
FIG. 2 illustrates that a typical source for the parasitic capacitance in an active-light-emitting diode (AMOLED) display is the capacitance that forms between coplanar anodes of the OLEDs.
A simplified model for how cross-talk occurs in a microdisplay is provided in FIG. 3. The circuit diagram on the left of FIG. 3 includes a pixel represented by OLED1, driven by an input signal VDRIVE from an off state (non-emitting or black) to an on state (emitting). It is coupled to an adjacent pixel represented by OLED2, via the parasitic capacitor CANODE.
The waveforms on the right of FIG. 3 show the drive signal applied to OLED1 as a step function, and the signal on the adjacent pixel OLED2 as a voltage spike. After the drive signal to OLED1 stabilizes at a high level, the signal on OLED2 starts to discharge via the resistance of the OLED diode. The resistance of the OLED diode is very high at low luminance levels so the discharge rate can be very long, resulting in a detectable amount of light from the adjacent pixel.